Method of fabricating a TFT-EL pixel

ABSTRACT

A method of making a 4-terminal active matrix electroluminescent device that utilizes an organic material as the electroluminescent medium is described. In this method, thin film transistors are formed from polycrystalline silicon at a temperature sufficiently low such that a low temperature, silica-based glass can be used as the substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned U.S. Ser. No. 08/355,742 entitled"TFT-EL Display Panel Using Organic Electroluminescent Media" by Tang etal and U.S. Ser. No. 08/355,786 entitled "An Electroluminescent DeviceHaving an Organic Electroluminescent Layer" by Tang et al, both filedconcurrently herewith, the disclosures of which are incorporated herein.

FIELD OF THE INVENTION

The present invention relates to a process for making a 4-terminalactive matrix thin-film-transistor electroluminescent device thatemploys organic material as the electroluminescent media.

INTRODUCTION

Rapid advances in flat-panel display (FPD) technologies have made highquality large-area, full-color, high-resolution displays possible. Thesedisplays have enabled novel applications in electronic products such aslap top computers and pocket-TVs. Among these FPD technologies, liquidcrystal display (LCD) has emerged as the display of choice in themarketplace. It also sets the technological standard against which otherFPD technologies are compared. Examples of LCD panels include: (1) 14",16-color LCD panel for work stations (IBM and Toshiba, 1989) (see K.Ichikawa, S. Suzuki, H. Matino, T. Aoki, T. Higuchi and Y. Oano, SIDDigest, 226 (1989)), (2) 6", full-color LCD-TV (Phillips, 1987) (see M.J. Powell, J. A. Chapman, A. G. Knapp, I. D. French, J. R. Hughes, A. D.Pearson, M. Allinson, M. J. Edwards, R. A. Ford, M. C. Hemmings, O. F.Hill, D. H. Nicholls and N. K. Wright, Proceeding, International DisplayConference, 63, 1987), (3) 4" full-color LCD TV (model LQ424A01, Sharp,1989) (see Sharp Corporation Technical Literature for model LQ424A01),and (4) 1 megapixel colored TFT-LCD (General Electric) (see D. E.Castleberry and G. E. Possin, SID Digest, 232 (1988)). All references,including patents and publications, are incorporated herein as ifreproduced in full below.

A common feature in these LCD panels is the use of thin-film-transistors(TFT) in an active-addressing scheme, which relaxes the limitations indirect-addressing (see S. Morozumi, Advances in Electronics and ElectronPhysics, edited by P. W. Hawkes, Vol. 77, Academic Press 1990). Thesuccess of LCD technology is in large part due to the rapid progress inthe fabrication of large-area TFT (primarily amorphous silicon TFT). Thealmost ideal match between TFT switching characteristics andelectrooptic LCD display elements also plays a key role.

A major drawback of TFT-LCD panels is they require bright backlighting.This is because the transmission factor of the TFT-LCD is poor,particularly for colored panels. Typically the transmission factor isabout 2-3 percent (see S. Morozumi, Advances in Electronics and,Electron Physics, edited by P. W. Hawkes, Vol. 77, Academic Press,1990). Power consumption for backlighted TFT-LCD panels is considerableand adversely affects portable display applications requiring batteryoperation.

The need for backlighting also impairs miniaturization of the flatpanel. For example, depth of the panel must be increased to accommodatethe backlight unit. Using a typical tubular cold-cathode lamp, theadditional depth is about 3/4 to 1 inch. Backlight also adds extraweight to the FPD.

An ideal solution to the foregoing limitation would be a low poweremissive display that eliminates the need for backlighting. Aparticularly attractive candidate isthin-film-transistor-electroluminescent (TFT-EL) displays. In TFT-ELdisplays, the individual pixels can be addressed to emit light andauxiliary backlighting is not required. A TFT-EL scheme was proposed byFischer in 1971 (see A. G. Fischer, IEEE Trans. Electron Devices, 802(1971)). In Fischer's scheme powdered ZnS is used as the EL medium.

In 1975, a successful prototype TFT-EL panel (6") was reportedly made byBrody et al. using ZnS as the EL element and CdSe as the TFT material(see T. P. Brody, F. C. Luo, A. P. Szepesi and D. H. Davies, IEEE Trans.Electron Devices, 22, 739 (1975)). Because ZnS-EL required a high drivevoltage of more than a hundred volts, the switching CdSe TFT element hadto be designed to handle such a high voltage swing. The reliability ofthe high-voltage TFT then became suspect. Ultimately, ZnS-based TFT-ELfailed to successfully compete with TFT-LCD. U.S. Patents describingTFT-EL technology include: U.S. Pat. Nos. 3,807,037; 3,885,196;3,913,090; 4,006,383; 4,042,854; 4,523,189; and 4,602,192.

Recently, organic EL materials have been devised. These materialssuggest themselves as candidates for display media in TFT-EL devices(see C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 51,913 (1987), C.W. Tang, S. A. VanSlyke and C. H. Chen, J. Appl. Phys., 65, 3610(1989)). Organic EL media have two important advantages: they are highlyefficient; and they have low voltage requirements. The lattercharacteristic distinguishes over other thin-film emissive devices.Disclosures of TFT-EL devices in which EL is an organic materialinclude: U.S. Pat. Nos. 5,073,446; 5,047,687, 5,059,861; 5,294,870;5,151,629; 5,276,380; 5,061,569; 4,720,432; 4,539,507; 5,150,006;4,950,950; and 4,356,429.

The particular properties of organic EL material that make it ideal forTFT are summarized as follows:

1) Low-voltage drive. Typically, the organic EL cell requires a voltagein the range of 4 to 10 volts depending on the light output level andthe cell impedance. The voltage required to produce a brightness ofabout 20 fL is about 5V. This low voltage is highly attractive for aTFT-EL panel, as the need for the high-voltage TFT is eliminated.Furthermore, the organic EL cell can be driven by DC or AC. As a resultthe driver circuity is less complicated and less expensive.

2) High efficiency. The luminous efficiency of the organic EL cell is ashigh as 4 lumens per watt. The current density to drive the EL cell toproduce a brightness of 20 fL is about 1 mA/cm². Assuming a 100% dutyexcitation, the power needed to drive a 400 cm² full-page panel is onlyabout 2.0 watts. The power need will certainly meet the portabilitycriteria of the flat panel display.

3) Low temperature fabrication. Organic EL devices can be fabricated atabout room temperature. This is a significant advantage compared withinorganic emissive devices, which require high-temperature (>300° C.)processing. The high-temperature processes required to make inorganic ELdevices can be incompatible with the TFT.

The simplest drive scheme for an organic EL panel is to have the organicdisplay medium sandwiched between two sets of orthogonal electrodes(rows and columns). Thus, in this two-terminal scheme, the EL elementserves both the display and switching functions. The diode-likenonlinear current-voltage characteristic of the organic EL elementshould, in principle, permit a high degree of multiplexing in this modeof addressing. However, there are several major factors limitingusefulness of the two-terminal scheme in connection with organic EL:

1) Lack of memory. The rise and decay time of the organic EL is veryfast, on the order of microseconds, and it does not have an intrinsicmemory. Thus, using the direct addressing method, the EL elements in aselected row would have to be driven to produce an instantaneousbrightness proportional to the number of scan rows in the panel.Depending on the size of the panel, this instantaneous brightness may bedifficult to achieve. For example, consider a panel of 1000 scan rowsoperating at a frame rate of 1/60 seconds. The allowable dwell time perrow is 17 μs. In order to obtain a time-averaged brightness of, forexample, 20 Fl, the instantaneous brightness during the row dwell timewould have to be a thousand times higher, i.e., 20,000 Fl, an extremebrightness that can only be obtained by operating the organic EL cell ata high current density of about 1 A/cm² and a voltage of about 15-20volts. The long-term reliability of a cell operating under these extremedrive conditions is doubtful.

2) Uniformity. The current demanded by the EL elements is supplied viathe row and column buses. Because of the instantaneous high current, theIR potential drops along these buses are not insignificant compared withthe EL drive voltage. Since the brightness-voltage characteristic of theEL is nonlinear, any variation in the potential along the buses willresult in a non-uniform light output.

Consider a panel with 1000 rows by 1000 columns with a pixel pitch of200μ×200μ and an active/actual area ratio of 0.5. Assuming the columnelectrode is indium tin oxide (ITO) of 10 ohms/square sheet (Ω/□)resistance, the resistance of the entire ITO bus line is at least 10,000ohms. The IR drop along this bus line for an instantaneous pixel currentof 800 μA (2 A/cm²) is more than 8 volts. Unless a constant currentsource is implemented in the drive scheme, such a large potential dropalong the ITO bus will cause unacceptable non-uniform light emission inthe panel. In any case, the resistive power loss in the bus is wasteful.A similar analysis can be performed for the row electrode bus that hasthe additional burden of carrying the total current delivered to theentire row of pixels during the dwell time, i.e., 0.8 A for the1000-column panel. Assuming a 1 μm thick aluminum bus bar of sheetresistance about 0.028 ohms/square the resultant IR drop is about 11volts, which is also unacceptable.

3) Electrode patterning. One set of the orthogonal electrodes, theanode-indium tin oxide, can be patterned by a conventionalphotolithographic method. The patterning of the other set of electrodeshowever, presents a major difficulty peculiar to the organic EL device.The cathode should be made of a metal having a work function lower than4 eV, and preferably magnesium alloyed with another metal such as silveror aluminum (see Tang et al., U.S. Pat. No. 4,885,432). Themagnesium-based alloy cathode deposited on top of the organic layerscannot be easily patterned by any conventional means involvingphotoresists. The process of applying the photoresist from an organicsolvent on the EL cell deleteriously affects the soluble organic layerunderneath the magnesium-based alloy layer. This causes delamination ofthe organic layers from the substrate.

Another difficulty is the extreme sensitivity of the cathode tomoisture. Thus, even if the photoresist can be successfully applied anddeveloped without perturbing the organic layers of the EL cell, theprocess of etching the magnesium-based alloy cathode in aqueous acidicsolution is likely to oxidize the cathode and create dark spots.

SUMMARY OF THE INVENTION

The present invention provides an active matrix 4-terminal TFT-EL devicein which organic material is used as the EL medium. The device comprisestwo TFTs, a storage capacitor and a light emitting organic EL padarranged on a substrate. The EL pad is electrically connected to thedrain of the second TFT. The first TFT is electrically connected to thegate electrode of the second TFT which in turn is electrically connectedto the capacitor so that following an excitation signal the second TFTis able to supply a nearly constant current to the EL pad betweensignals. The TFT-EL devices of the present invention are typicallypixels that are formed into a flat panel display, preferably a displayin which the EL cathode is a continuous layer across all of the pixels.

The TFT-organic EL device of the present invention are formed in amulti-step process as described below:

A first thin-film-transistor (TFT1) is disposed over the top surface ofthe substrate. TFT1 comprises a source electrode, a drain electrode, agate dielectric, and a gate electrode; and the gate electrode comprisesa portion of a gate bus. The source electrode of TFT1 is electricallyconnected to a source bus.

A second thin-film-transistor (TFT2) is also disposed over the topsurface of the substrate, and TFT2 also comprises a source electrode, adrain electrode, a gate dielectric, and a gate electrode. The gateelectrode of TFT2 is electrically connected to the drain electrode ofthe first thin-film-transistor.

A storage capacitor is also disposed over the top surface of thesubstrate. During operation, this capacitor is charged from anexcitation signal source through TFT1, and discharges during the dwelltime to provide nearly constant potential to the gate electrode of TFT2.

An anode layer is electrically connected to the drain electrode of TFT2.In typical applications where light is emitted through the substrate,the display is a transparent material such as indium tin oxide.

A dielectric passivation layer is deposited over at least the source ofTFT1, and preferably over the entire surface of the device. Thedielectric passivation layer is etched to provide an opening over thedisplay anode.

An organic electroluminescent layer is positioned directly on the topsurface of the anode layer. Subsequently, a cathode layer is depositeddirectly on the top surface of the organic electroluminescent layer.

In preferred embodiments, the TFT-EL device of the present invention ismade by a method using low pressure and plasma enhanced chemical vapordeposition combined with low temperature (i.e. less than 600° C.)crystallization and annealing steps, hydrogen passivation andconventional patterning techniques.

The thin-film-transistors are preferably formed simultaneously by amulti-step process involving:

the deposition of silicon that is patterned into polycrystalline siliconislands;

chemical vapor deposition of a silicon dioxide gate electrode; and

deposition of another polycrystalline silicon layer which is patternedto form a self-aligned gate electrode so that after ion-implantation asource, drain, and gate electrode are formed on eachthin-film-transistor.

The construction of pixels having thin-film-transistors composed ofpolycrystalline silicon and silicon dioxide provides improvements indevice performance, stability, reproducibility, and process efficiencyover other TFTs. In comparison, TFTs composed of CdSe and amorphoussilicon suffer from low mobility and threshold drift effect.

There are several important advantages in the actual panel constructionand drive arrangement of a TFT-organic EL device of the presentinvention:

1) Since both the organic EL pad and the cathode are continuous layers,the pixel resolution is defined only by the feature size of the TFT andthe associated display ITO pad and is independent of the organiccomponent or the cathode of the EL cell.

2) The cathode is continuous and common to all pixels. It requires nopatterning for pixel definition. The difficulty of patterning thecathode in the two-terminal scheme is therefore eliminated.

3) The number of scanning rows is no longer limited by the short rowdwell time in a frame period, as the addressing and excitation signalsare decoupled. Each scan row is operated at close to 100% duty factor.High resolution can be obtained since a large number of scan rows can beincorporated into a display panel while maintaining uniform intensity.

4) The reliability of the organic EL element is enhanced since itoperates at a low current density (1 mA/cm²) and voltage (5V) in a 100%duty factor.

5) The IR potential drops along the buses are insignificant because ofthe use of a common cathode and the low current density required todrive the EL elements. Therefore the panel uniformity is notsignificantly affected by the size of the panel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an active matrix 4-terminal TFT-ELdevice. T1 and T2 are thin-film-transistors, Cs is a capacitor and EL isan electroluminescent layer.

FIG. 2 is a diagrammatic plan view of the 4-terminal TFT-EL device ofthe present invention.

FIG. 3 is a cross-sectional view taken along the line A-A' in FIG. 2.

FIG. 4 is a cross-sectional view taken along the line A-A', illustratingthe process of forming a self-aligned TFT structure for ionimplantation.

FIG. 5 is a cross-sectional view taken along the line A-A', illustratingthe processing steps of depositing a passivation oxide layer and openingcontact cuts to the source and drain regions of thethin-film-transistor.

FIG. 6 is a cross-sectional view taken along line A-A', illustratingdeposition of an aluminum electrode.

FIG. 7 is a cross-sectional view taken along line A-A', illustratingdeposition of the display anode and a passivation layer that has beenpartially etched from the surface of the display anode.

FIG. 8 is a cross-sectional view taken along line A-A', illustrating thesteps of depositing an electroluminescent layer and a cathode.

FIG. 9 is a cross-sectional view taken along line B-B' in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the schematic of an active matrix 4-terminal TFT-EL displaydevice. Each pixel element includes two TFTs, a storage capacitor and anEL element. The major feature of the 4-terminal scheme is the ability todecouple the addressing signal from the EL excitation signal. The ELelement is selected via the logic TFT (T1) and the excitation power tothe EL element is controlled by the power TFT (T2). The storagecapacitor enables the excitation power to an addressed EL element tostay on once it is selected. Thus, the circuit provides a memory thatallows the EL element to operate at a duty cycle close to 100%,regardless of the time allotted for addressing.

The construction of the electroluminescent device of the presentinvention is illustrated in FIGS. 2 and 3. The substrate of this deviceis an insulating and preferably transparent material such as quartz or alow temperature glass. The term transparent, as it is used in thepresent disclosure, means that the component transmits sufficient lightfor practical use in a display device. For example, componentstransmitting 50% or more of light in a desired frequency range areconsidered transparent. The term low temperature glass refers to glassesthat melt or warp at temperatures above about 600° C.

In the TFT-EL device illustrated in FIG. 2, TFT1 is the logic transistorwith the source bus (column electrode) as the data line and the gate bus(row electrode) as the gate line. TFT2 is the EL power transistor inseries with the EL element. The gate line of TFT2 is connected to thedrain of TFT1. The storage capacitor is in series with TFT1. The anodeof the EL element is connected to the drain of TFT2.

The construction of the TFT-EL of FIG. 2 is shown in cross-sectionalview in FIGS. 3-9. The cross-sectional views shown in FIGS. 3-8 aretaken along section line A-A' in FIG. 2. The cross-sectional view inFIG. 9 is taken along line B-B' in FIG. 2.

In the first processing step, a polysilicon layer is deposited over atransparent, insulating substrate and the polysilicon layer is patternedinto an island (see FIG. 4) by photolithography. The substrate may becrystalline material such as quartz, but preferably is a less expensivematerial such as low temperature glass. When a glass substrate isutilized, it is preferable that the entire fabrication of the TFT-EL becarried out at low processing temperatures to prevent melting or warpingof the glass and to prevent out-diffusion of dopants into the activeregion. Thus, for glass substrates, all fabrication steps should beconducted below 1000° C. and preferably below 600° C.

Next, an insulating gate material 42 is deposited over the polysiliconisland and over the surface of the insulating substrate. Insulatingmaterial is preferably silicon dioxide that is deposited by a chemicalvapor deposition (CVD) technique such as plasma enhanced CVD (PECVD) orlow pressure CVD (LPCVD). Preferably, the gate oxide insulating layer isabout 1000Å in thickness.

In the next step, a layer of silicon 44 is deposited over the gateinsulator layer and patterned by photolithography over the polysiliconisland such that after ion implantation, source and drain regions areformed in the polysilicon island. The gate electrode material ispreferably polysilicon formed from amorphous silicon. Ion implantationis conducted with N-type dopants, preferably arsenic. The polysilicongate electrode also serves as the bottom electrode of the capacitor (seeFIG. 9).

In a preferred embodiment of the present invention, the thin filmtransistors do not utilize a double gate structure. Thus manufacturingis made less complex and less expensive.

A gate bus 46 is applied and patterned on the insulating layer. The gatebus is preferably a metal silicide such as tungsten silicide (WSi₂).

In the next step, an insulating layer, preferably silicon dioxide, 52 isapplied over the entire surface of the device.

Contact holes 54 and 56 are cut in the second insulating layer (see FIG.5) and electrode materials are applied to form contacts with thethin-film-transistors (see FIGS. 6 and 7). The electrode material 62attached to the source region of TFT2 also forms the top electrode ofthe capacitor (see FIG. 9). A source bus and ground bus are also formedover the second insulating layer (see FIG. 2). In contact with the drainregion of TFT2 is a transparent electrode material 72, preferably ITO,which serves as the anode for the organic electroluminescent material.

In the next step, a passivating layer 74 of an insulating material,preferably silicon dioxide, is deposited over the surface of the device.The passivation layer is etched from the ITO anode leaving a taperededge 76 which serves to improve the adhesion of the subsequently appliedorganic electroluminescent layer. A tapered edge is necessary to producereliable devices because the present invention utilizes relatively thinorganic EL layers, typically 150 to 200 nm thick. The passivation layeris typically about 0.5 to about 1 micron thick. Thus, if the edge of thepassivation layer forms a perpendicular or sharp angle with respect tothe anode layer, defects are likely to occur due to discontinuities inthe organic EL layer. To prevent defects the passivation layer shouldhave a tapered edge. Preferably the passivation layer is tapered at anangle of 10 to 30 degrees with respect to the anode layer.

The organic electroluminescent layer 82 is then deposited over thepassivation layer and the EL anode layer. The materials of the organicEL devices of this invention can take any of the forms of conventionalorganic EL devices, such as those of Scozzafava EPA 349,265 (1990); TangU.S. Pat. No. 4,356,429; VanSlyke et al. U.S. Pat. No. 4,539,507;VanSlyke et al. U.S. Pat. No. 4,720,432; Tang et al. U.S. Pat. No.4,769,292; Tang et al. U.S. Pat. No. 4,885,211; Perry et al. U.S. Pat.No. 4,950,950; Littman et al. U.S. Pat. No. 5,059,861; VanSlyke U.S.Pat. No. 5,047,687; Scozzafava et al. U.S. Pat. No. 5,073,446; VanSlykeet al. U.S. Pat. No. 5,059,862; VanSlyke et al. U.S. Pat. No. 5,061,617;VanSlyke U.S. Pat. No. 5,151,629; Tang et al. U.S. Pat. No. 5,294,869;and Tang et al. U.S. Pat. No. 5,294,870, the disclosures of which areincorporated by reference. The EL layer is comprised of an organic holeinjecting and transporting zone in contact with the anode, and anelectron injecting and transporting zone forming a junction with theorganic hole injecting and transporting zone. The hole injecting andtransporting zone can be formed of a single material or multiplematerials, and comprises a hole injecting layer in contact with theanode and a contiguous hole transporting layer interposed between thehole injecting layer and the electron injecting and transporting zone.Similarly, the electron injecting and transporting zone can be formed ofa single material or multiple materials, and comprises an electroninjecting layer in contact with the cathode and a contiguous electrontransporting layer that is interposed between the electron injectinglayer and the hole injecting and transporting zone. Recombination of theholes and electrons, and luminescence, occurs within the electroninjecting and transporting zone adjacent the junction of the electroninjecting and transporting zone and the hole injecting and transportingzone. The components making up the organic EL layer are typicallydeposited by vapor deposition, but may also be deposited by otherconventional techniques.

In a preferred embodiment the organic material comprising the holeinjecting layer has the general formula: ##STR1## wherein: Q is N orC(R)

M is a metal, metal oxide or metal halide

R is hydrogen, alkyl, aralkyl, aryl or alkaryl, and

T, and T₂ represent hydrogen or together complete an unsaturated sixmembered ring that can include substituents such as alkyl or halogen.Preferred alkyl moieties contain from about 1 to 6 carbon atoms whilephenyl constitutes a preferred aryl moiety.

In a preferred embodiment the hole transporting layer is an aromatictertiary amine. A preferred subclass of aromatic tertiary amines includetetraaryldiamines having the formula: ##STR2## wherein Are is an arylenegroup,

n is an integer from 1 to 4, and

Ar, R₇, R₈ and R₉ are independently selected aryl groups.

In a preferred embodiment, the luminescent, electron injecting andtransporting zone contains a metal oxinoid compound. A preferred exampleof a metal oxinoid compound has the general formula: ##STR3## wherein R₁-R₇ represent substitutional possibilities. In another preferredembodiment, the metal oxinoid compound has the formula: ##STR4## whereinR₂ -R₇ are as defined above and L₁ -L₅ collectively contain twelve orfewer carbon atoms and each independently represent hydrogen orhydrocarbon groups of from 1 to 12 carbon atoms, provided that L₁ and L₂together or L₂ and L₃ together can form a fused benzo ring. In anotherpreferred embodiment, the metal oxinoid compound has the formula:##STR5## wherein R₂ -R₆ represent hydrogen or other substitutionalpossibilities.

The foregoing examples merely represent some preferred organic materialsused in the electroluminescent layer. They are not intended to limit thescope of the invention, which is directed to organic electroluminescentlayers generally. As can be seen from the foregoing examples, theorganic EL material includes coordination compounds having organicligands. The TFT-EL device of the present invention does not includepurely inorganic materials such as ZnS.

In the next processing step, the EL cathode 84 is deposited over thesurface of the device. The EL cathode may be any electronicallyconducting material, however it is preferable that the EL cathode bemade of a material having a work function of less than 4 eV (see Tang etal. U.S. Pat. No. 4,885,211). Low work function metals are preferred forthe cathode since they readily release electrons into the electrontransporting layer. The lowest work function metals are the alkalimetals; however, their instability in air render their use impracticalin some situations. The cathode material is typically deposited byphysical vapor deposition, but other suitable deposition techniques areapplicable. A particularly desirable material for the EL cathode hasbeen found to be a 10:1 (atomic ratio) magnesium:silver alloy.Preferably, the cathode is applied as a continuous layer over the entiresurface of the display panel. In another embodiment, the EL cathode is abilayer composed of a lower layer of a low work function metal adjacentto the organic electron injecting and transporting zone and, overlyingthe low work function metal, a protecting layer that protects the lowwork function metal from oxygen and humidity. Optionally, a passivationlayer may be applied over the EL cathode layer.

Typically, the anode material is transparent and the cathode materialopaque so that light is transmitted through the anode material. However,in an alternative embodiment, light is emitted through the cathoderather than the anode. In this case the cathode must be lighttransmissive and the anode may be opaque. A practical balance lighttransmission and technical conductance is typically in the thicknessrange of 5-25 nm.

A preferred method of making a thin-film-transistor according to thepresent invention is described below. In a first step, an amorphoussilicon film of 2000±20Å thickness is deposited at 550° C. in an LPCVDsystem with silane as the reactant gas at a process pressure of 1023mTorr. This is followed by a low temperature anneal at 550° C. for 72hours in vacuum to crystallize the amorphous silicon film into apolycrystalline film. Then a polysilicon island is formed by etchingwith a mixture of SF₆ and Freon 12 in a plasma reactor. Onto thepolysilicon island active layer is deposited a 1000±20Å PECVD SiO₂ gatedielectric layer. The gate dielectric layer is deposited from a 5/4ratio of N₂ O/SiH₄ in a plasma reactor at a pressure of 0.8 Torr with apower level of 200W and a frequency of 450 KHz at 350° C. for 18minutes.

In the next step an amorphous silicon layer is deposited over the PECVDgate insulating layer and converted to polycrystalline silicon using thesame conditions as described above for the first step. A photoresist isapplied and the second polysilicon layer is etched to form aself-aligned structure for the subsequent ion implantation step. Thesecond polysilicon layer is preferably about 3000Å thick.

Ion implantation is conducted by doping with arsenic at 120 KeV at adose of 2×10¹⁵ /cm² to simultaneously dope the source, drain and gateregions. Dopant activation is carried out at 600° C. for two hours in anitrogen atmosphere.

In the next step, a 5000Å thick silicon dioxide layer is deposited byconventional low temperature methods. Aluminum contacts are formed by aphysical vapor deposition and sintered in forming gas (10% H₂, 90% N₂)for thirty minutes at 400° C.

Finally, hydrogen passivation of the thin-film-transistor is carried outin an electron cyclotron resonance reactor (ECR). ECR hydrogen plasmaexposure is conducted at a pressure of 1.2×10⁻⁴ Torr with a microwavepower level of 900W and a frequency of 3.5 GHz. Hydrogen passivation isperformed for fifteen minutes at a substrate temperature of 300° C. Thisprocedure results in a thin-film-transistor device having a lowthreshold voltage, a high effective carrier mobility and an excellenton/off ratio.

As an example of characteristics of the present invention, consider thedrive requirements for the following TFT-EL panel:

    ______________________________________                                        Number of rows     = 1000                                                     Number of columns  = 1000                                                     Pixel dimension    = 200 μm × 200 μm                              EL fill-factor     = 50%                                                      frame time         = 17 ms                                                    row dwell time     = 17 μs                                                 Avg brightness     = 20 fL                                                    EL pixel current   = 0.8 μA                                                Duty cycle         = 100%                                                     EL power source    = 10 v rms                                                 ______________________________________                                    

These drive requirements are met by the following specifications for theTFTs and the storage capacitor:

    ______________________________________                                        TFT1                                                                          ______________________________________                                        Gate voltage  = 10 V                                                          Source voltage                                                                              = 10 V                                                          On-current    = 2 μA                                                       Off-current   = 10.sup.-11 A TFT2                                             Gate voltage  = 10 V                                                          Source voltage                                                                              = 5 V                                                           On-current    = 2 × EL pixel current                                                  = 1.6 μA                                                     Off-current   = 1 nA                                                          Storage capacitor:                                                            Size          = 1 pf                                                          ______________________________________                                    

The on-current requirement for TFT1 is such that it is large enough tocharge up the storage capacitor during the row dwell time (17 μs) to anadequate voltage (10V) in order to turn on the TFF2. The off-currentrequirement for TFT1 is such that it is small enough that the voltagedrop on the capacitor (and TFT2 gate) during the frame period (17 ms) isless than 2%.

The on-current requirement for TFT2 is (designed to be) about 2 timesthe EL pixel current, 1.6 μA. This factor of two allows for adequatedrive current to compensate for the gradual degradation of the organicEL element with operation. The off-current of TFT2 affects the contrastof the panel. An off-current of 1 nA should provide an on/off contrastratio greater than 500 between a lit and an unlit EL element. The actualcontrast ratio of the panel may be lower, depending on the ambientlighting factor.

For a full page panel of 400 cm² the power required by the EL elementsalone is about 4 watts. ##EQU1## This power consumption excludes thepower consumed by the TFTs. Since TFT2 is in series with the EL element,any source-drain voltage drop across TFT2 will result in substantialpower loss in the TFT2. Assuming a source-drain voltage of 5 volts, thetotal power loss on TFT2 is 2 watts. The power consumption for TFT1 isestimated to be no greater than 1 watt for the 1000×1000 panel. Thepower needed for the row (gate) drivers is negligible, on the order of afew tens of milliwatts, and the power for the column (source) drivers ison the order of 0.5 watt (see S. Morozumi, Advances in Electronics andElectron Physics, edited by P. W. Hawkes, Vol. 77, Academic Press,1990). Thus, the total power consumption for a full page TFT-EL panel isabout 7 watts. Realistically, the average power consumption would bemuch less since the EL screen is not 100% on in average usage.

The TFT-EL panel of the present invention has two important advantagesin terms of power requirements over TFT-LCD panels. First, the TFT-ELpower need is relatively independent of whether the panel is monochromeor multi-color, provided that the color materials have a similarluminescent efficiency. In contrast, the TFT-LCD colored panel requiresa much higher power than the monochrome panel because the transmissionfactor is greatly reduced in the colored panel by the color filterarrays. Second, the LCD backlight has to stay on regardless of thescreen usage factor. In contrast, the TFT-EL power consumption is highlydependent on this usage factor. The average power consumption is muchless since less than 100% of the EL screen is emitting at any given timein typical applications.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

Parts List

42 gate material

44 silicon layer

46 gate bus

52 insulating layer

54 contact holes

56 contact holes

62 electrode material

72 electrode material

74 passivating layer

76 tapered edge

82 EL layer

84 EL cathode

We claim:
 1. A method of fabricating a TFT-EL pixel comprising the stepsof:a) providing an insulating substrate having top and bottom surfaces,depositing a layer of silicon on the top surface of said substrate andpatterning said layer to form a first and a second polycrystallinesilicon island; b) depositing a first dielectric layer over the topsurface of said substrate and over said first and second polycrystallineislands to form a gate dielectric layer; c) depositing a first strip ofmaterial over said first polycrystalline silicon island, and depositinga second strip of material over said second polycrystalline siliconisland; d) depositing a layer for the bottom electrode of a capacitor;e) ion-implanting into said polycrystalline silicon islands and saidstrips of material to form source and drain regions and a doped gateelectrode; thus forming first and second thin-film-transistors; f)depositing a second dielectric layer covering said first dielectriclayer, said strips of material, and said bottom layer of a capacitor; g)etching through said first and second dielectric layers to form sourceand drain contact holes and depositing conducting material into saidsource and drain contact holes; h) depositing a conductive layer thatforms the top electrode of said capacitor; i) depositing a display anodelayer electrically connected to said drain of said secondthin-film-transistor; said anode layer disposed on said seconddielectric layer; j) depositing a third dielectric layer over thesurface of the article resulting from step i; k) etching a hole throughsaid third dielectric layer to expose said display anode layer; l)depositing an organic electroluminescent layer over said display anodelayer; and depositing a cathode layer over said organic layer.
 2. Themethod of claim 1 additionally comprising the steps of depositing asource bus electrically connected to said source region of said firstthin-film-transistor; and depositing a ground bus electrically connectedto said capacitor.
 3. The method of claim 1 wherein said first strip ofmaterial is selected from the group consisting of silicon and a metalsilicide, and said second strip of material is silicon.
 4. The method ofclaim 1 wherein said step of etching through said third dielectric layercreates a tapered edge on said third dielectric layer.
 5. The method ofclaim 1 wherein said polycrystalline silicon islands are formed on a lowtemperature glass substrate by the chemical vapor deposition of anamorphous silicon layer and a low temperature anneal at less than 600°C., followed by photo lithographically forming islands in a plasma etch.6. The method of claim 5 wherein said second strip of material and saidbottom electrode of said capacitor are formed of a common polysiliconlayer that is doped during said ion-implanting step with a dopantselected from the group consisting of Sb, As, P and N.
 7. The method ofclaim 6 wherein said ion-implanting step is followed by dopantactivation conducted at about 600° C.
 8. The method of claim 6 whereinsaid thin-film-transistors are subjected to a hydrogen passivation stepin an electron cyclotron resonance plasma system at about 300° C.
 9. Themethod of claim 1 wherein said organic electroluminescent layercomprises an electron transporting and injecting zone and a holeinjecting and transporting zone wherein both zones are deposited byvacuum evaporation.
 10. The method of claim 1 wherein said organicelectroluminescent layer is about 150 to 200 nm thick.